Semiconductor lighting systems that include the now familiar LED, are growing rapidly in popularity and success. Semiconductor lighting presents a number of advantages over more common lighting systems, such as incandescent and fluorescent lighting systems; they are more energy efficient than incandescent lamps, and they exhibit significantly greater lamp life than either incandescent or fluorescent lamps.
However, the current state of semiconductor lighting products shows that they nevertheless suffer from a number of disadvantages when compared to incandescent and fluorescent lighting systems, mainly when dimming the lamp. Employing LED lighting devices in conjunction with conventional AC dimming circuits has been addressed in a number of ways that are generally unsatisfactory in the marketplace due to the design of dimmers in existing installations.
The most commonly installed type of dimming circuit incorporates a 300 or 600-watt triac dimmer. These dimmers are designed to operate on lamp circuits with non-trivial resistive loads, such as an incandescent or halogen lamp, and consequently require a holding current in order to function. Triac dimmers remove some portion of the AC voltage waveform in order to dim lamps effectively. This is an efficient and satisfactory method to achieve dimming incandescent and halogen lamp circuits, where the thermal inertia of the filament maintains a steady illumination; but this is an unsatisfactory for LED lighting implementations.
The LEDs employed in LED lamp packages may be either high current or low current LEDs. LED lamp packages incorporating high current LEDs rely on a conventional converter circuit (such as a buck or flyback converter) to bring the line voltage down to the operating voltage of the LEDs. This approach is currently predominant in the art. Even in non-dimming applications, the converter circuitry requires several components including bulky inductive elements. Each component in turn adds to energy waste and reduced reliability through the circuit. Moreover, each additional component increases the size of the driver circuit, generally making it more difficult to fit existing incandescent fixtures, sockets, or form factors. The bulkiest of these components are the inductive elements needed as energy storage elements for high current LED drivers. The higher cost associated with the number of components required by high current LED drivers also contributes to a high cost that may result in a low market penetration for LED lamps.
Two significant issues occur when such converter circuitry is placed in a triac-dimmed circuit.
First, the conventional converter circuitry draws no current at all at some portions of power line cycle, a situation incompatible with the triac dimmer requirement for a holding current. This generally results in unacceptable lamp pulsing and flickering that is likely to annoy the end user.
Secondly, the conventional converter circuitry may draw high peak current with having a high total harmonic distortion (THD) and low power factor that inordinately limit number of LED lamps can be connected to the dimmer.
In short, conventional converter circuitry is not acceptable for dimming applications of LEDs. In order to obtain satisfactory dimming performance in high current LED lighting systems, even more components are required. These solutions exacerbate rather than alleviate the problems of efficiency, cost, size, and reliability.
This poor performance of high current LEDs in dimming applications has led leading manufacturers of energy efficient lighting products to turn to low current LEDs for its dimming applications. Low current LEDs avoid many of the problems of high power LEDs in a dimming circuit. Current dimmable low current LED lamps incorporate low current LEDs in one or more series circuits and a driver circuit that allows the lamps to operate on standard line power. These driver circuits are generally comprised of a bridge rectifier and a capacitor to smooth the rectifier output. However, the inclusion of the capacitor may cause the lamp to draw too little current to maintain a holding current from an industry standard triac dimmer, initially extinguishing the lamp and then causing an unacceptable flicker similar to that found in a high current LED lamp on a dimmed conventional converter circuit. For low current LEDs, commonly, the lamp cycles rapidly from illumination to extinguishments, as the capacitor in the driver circuit charges sufficiently to sustain LED illumination, removes the holding current from the triac subsequently discharges, begins to draw again on the triac, and re-illuminates the LED at full power. These voltage spikes cause drastic flickering that is unacceptable in general lighting applications.
Flickering in low current LED lamp packages is currently overcome by the addition of a load resistor to the driver circuit before the bridge rectifier to dissipate sufficient power to maintain a holding current. However, the inclusion of the resistor may reduce the efficiency of the lamp by 50% or more, depending on the output of the lamp. Adding a load resistor across the line is an improvement, but remains an unsatisfactory solution.
The following prior art discloses the various aspects in the design of an LED driver using MOSFETs as the primary source to energize the LEDs.
U.S. Pat. No. 5,204,563, granted Apr. 20, 1993, to B. L. Jason, discloses a MOSFET output driver circuit that is protected from overstress caused by commutating currents. The MOSFETs are protected by employing a gate control circuit and a small inductor in series with the sources of the MOSFETs. The circuit limits the rate of change of current that reverse biases a MOSFET's drain-source diode. The circuit is applicable to totem-pole and bridge configurations.
U.S. Pat. No. 7,550,934, granted Jun. 23, 2009, to Q. Deng, et al., discloses an LED driver that drives one or more strings of series-connected LEDs. A feedback voltage at a sense resistor is detected by an op amp, and the op amp controls the conductivity of a MOSFET in series with the LEDs to regulate the peak current. The MOSFET is also controlled by a PWM brightness control signal to turn the LEDs on and off at the PWM duty cycle. A boost regulator provides an output voltage to the string of LEDs. A divided voltage at the end of the string of LEDs is regulated by the boost controller to keep the divided voltage constant. When an LED becomes an open circuit, the boost regulator controller is immediately decoupled from the regulator's switching transistor. If an LED shorts, the boost regulator reduces its output voltage, and the duty cycle of the brightness control signal is automatically increased.
U.S. Pat. No. 7,830,094, granted Nov. 9, 2010, to X. Y. He, et al., discloses a driver arrangement for LED light sources that includes a transformer having primary and secondary winding connections. A first electronic switch (SW1) controls current flow through the primary winding, and a sensing resistor is coupled to the primary winding to produce a current sensing signal. A second electronic switch (SW2) is sensitive to the sensing signal to switch off SW1 as current flow through the primary winding reaches a given threshold. SW1 is thus alternatively turned-on and off in alternating turn-on and switch-off phases to power the LED light source via the secondary winding. SW1 is a field effect transistor, preferably a MOSFET. The transformer is SELV-rated. The sensing resistor may be a variable resistor to adjust the duration of the on/off phases to permit controlled dimming.
The following prior art discloses the various aspects in the design of an LED driver using a triac semiconductor as a voltage dropping element.
U.S. Pat. No. 4,580,080, granted Apr. 1, 1986, to A. M. Smith, discloses a phase control ballast in which a reactor and a triac are connected in series with a high intensity discharge (hid) lamp across an ac voltage source. A supra-linear converter connected to a rectifier-filter provides a reference voltage which is a supra-linear function of the source voltage. A ramp generator provides a ramp voltage climbing at a constant rate. At the instant when the ramp voltage exceeds the level of the reference voltage, a comparator circuit provides a signal to the gate of the triac which turns it on. A triac state detector responds to the turning on of the triac in either polarity by dropping the ramp voltage to zero and holding it at zero until the triac turns itself off.
It is therefore an object of the present invention to provide a dimmable lighting system utilizing serially connected low current LEDs.
It is another object of the present invention to provide a dimmable lighting system that does not utilize inductive components which in turn adds to energy waste and decreased circuit reliability and increased product size.
It is another object of the present invention to provide a dimmable semiconductor lamp driver providing a dynamic holding current to flow through the triac dimming circuit
It is still another object of the present invention to provide a dimmable semiconductor lamp driver providing flicker-free dimming.
It is still yet another object of the present invention to provide a dimmable semiconductor lamp driver providing flicker-free dimming, while attaining a 0-100% dimming range.
Another object of the present invention is to provide a dimmable semiconductor lamp driver with improved thermal protection.
An additional object of the present invention is to provide two closed loop negative feedback control systems to regulate the luminescent output of the LED array and to control the holding current flowing through the triac dimming circuit.
It is a final object of the present invention to provide a dimmable semiconductor lamp driver with having increased energy and manufacturing efficiencies.
The preceding and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of the detailed description provided herein with appropriate reference to the accompanying drawings.